The printable tattoo paper is a two-layer film-coated paper. A glossy, transparent and extensible film is coated on a backboard paper on the other side. The backboard paper can be removed by dampening the backboard. In this experiment, grids were printed on the glossy side of a tattoo paper using a high-resolution inkjet printer (5,760 × 1440 dpi). A grid tab was then trimmed and fastened (glossy side face down) on an aluminum specimen by using cyanoacrylate adhesive. The backboard paper was then dampened and removed, and the grids were shown on the specimen. A series of pictures of the grids were taken during the tensile test by using a 5-mega pixel digital camera (2,560 × 1920 pixels), and transferred to a computer. A photo-image management software was employed to measure the pixel-coordinates of the grids, and the strains were calculated by dividing the grid extensions by the original grid length. The strains measured by grids were then compared with those measured by strain-gage and those measured by scrape-lines. The results show that, if measured with standard procedures, the accuracy of grid strains is within ±10% when the measured strains exceed 0.0025 (2500με). Even if there is an error of ±1 pixel (the worst scenario) in the grid extension measurements, the accuracy of grid strains is still within ±10% when the measured strains exceed 0.005 (5000με). Therefore, the strain measurement using grid tabs can provide an accurate method for measuring large strains, which cannot be measured by strain-gages. This method overcomes the strain-gage limitations and can be used to succeed the strain-gage to continuously measure the large strains until the specimen breaks.
